Our previous studies have demonstrated that absorption of Indium-111 labeled DTPA occurs at a higher rate in central (airway dominated) lung zones of CF subjects compared to controls (42 vs. 32 %/hr, CF n= 9, control n=10, p=0.03). We believe that this increased DTPA absorption is being caused by the increased liquid absorption occurring in these airways, however there are other potential causes such as increase in tight junction permeability or epithelial denuding.